Field of the Invention
This invention relates generally to a hierarchical sandwich or box structure and, more particularly, to a sandwich box impact beam for a vehicle that includes spaced apart micro-truss sandwich structures defining an open area therebetween.
Discussion of the Related Art
Modern vehicles are equipped with a number of impact beams providing structural integrity against collisions and impacts with other objects, such as other vehicles. More particularly, impact beams are traditionally used in vehicle designs to protect occupants from front, side and/or rear impacts by absorbing energy through deformation in the event of a vehicle crash and distributing the applied dynamic loads to other energy absorbing sub-systems on the vehicle. For example, it is known to provide impact beams in a front energy management or bumper assembly, a rear energy management or bumper assembly and side impact assemblies on a vehicle. Impact beams at the front and rear of the vehicle are usually referred to as bumper beams, and impact beams on the sides of the vehicle are sometimes referred to as anti-intrusion bars. In all cases, it is desirable to provide an impact beam with low mass, high flexural stiffness and strength, and high energy absorption per unit mass. The lightweight requirement is predicated by fuel economy standards and the fact that impact beams are located both very close to and very far from the vehicle's center of mass. Maximizing the flexural stiffness and strength is necessary if the beam is to survive low speed impacts without damage and transfer impact loads throughout the duration of a high speed impact event. Further, a high level of energy absorption translates into reduced load transfer to the occupants of the vehicle, thus increasing safety.
In one known vehicle front energy management system, an impact beam is comprised of a top and bottom facesheet in combination with an internal structural core for providing high energy impact resistance in a light weight and cost effective manner. Typically, the impact beam for such a system includes aluminum, steel, carbon fiber, etc. layers that are extruded, roll-formed, etc. A hard energy absorbing layer may be formed on the impact beam having the general shape of an outer fascia trim panel. A soft energy absorbing layer is then formed on the hard energy absorbing layer and the front fascia panel is then provided over the soft energy absorbing layer. The combination of the hard energy absorbing layer and the soft energy absorbing layer provides a transition between the impact beam and the front fascia panel so as to allow the system to conform to the desired shape of the front fascia panel which may have significant angles and forms required by the vehicle styling. The hard energy absorbing layer and the soft energy absorbing layer also provide a transition between the fascia panel and the impact beam to effectively absorb low speed impacts without significantly compromising system integrity.
It is known in the art to provide vehicle impact beams having sandwich structures. These prior art impact beams can generally be categorized into three designs, namely, hollow beams that are fully or partially reinforced with a polymer or metallic foam, single or dual-sided facesheets reinforced with a honeycomb-like cellular core, and formed composite impact beams. For hollow metallic or polymer matrix composite tube structures which are fully or partially reinforced with a lightweight foam core, the material used for the core can be either a metallic or polymeric foam that is bonded, mechanically attached or interference fit into the tube structure. The purpose of the core is to carry shear loads in the sandwich structure and absorb energy in the event of a low or high speed impact, which is a distinction dependent on the density and composition of the foam. The use of honeycomb or honeycomb-like ordered cellular cores to provide reinforcement to one or two flat facesheets have an open-sided sandwich designs and have honeycomb, discrete-stiffened or wine-crate structures extending from the front face of the impact beam back towards the passenger compartment of the vehicle. If a second facesheet is not included between the core and the passenger compartment, then the core material must be relatively dense since it provides most of the flexural stiffness to the structure adjacent to the shear load transfer.
For continuous or discontinuous fiber reinforced polymer matrix composite impact beams, the matrix material may either be a thermoplastic or thermoset polymer introduced via resin transfer molding, compression molding, blow molding, or other similar fabrication processes.
It is also known in the art to fabricate a three-dimensional network of photopolymer waveguides comprising a unitary truss or lattice architecture, hereafter referred to equivalently as micro-truss or micro-lattice. For example, U.S. Pat. Nos. 7,653,279 and 7,382,959 disclose a process for fabricating such a micro-truss structure. Generally, the process includes providing a reservoir or mold filled with a volume of a curable monomer and covered by a mask including strategically positioned apertures. UV light sources are positioned relative to the mask and exposure to collimated UV light through the mask apertures forms a series of interconnected self-propagating photopolymer waveguides, referred to herein as struts, to form the truss or lattice architecture. Once the photopolymer waveguides are formed, the reservoir is emptied of the unpolymerized monomer which was not exposed to UV light. The micro-truss structure may then undergo a post-cure operation to increase the cross-link density in the photopolymer waveguides. This post-cure may be accomplished via a thermal cure, an additional exposure to UV light, an equivalent method or combination thereof.